Conventionally, a SiC semiconductor device having a vertical type MOSFET as a transistor with a vertical type trench gate structure is disclosed (for example, Patent Document No. 1). The vertical type MOSFET has a structure such that a P conductive type base region is formed on a N− conductive type drift layer, a N+ conductive type source region is formed in a surface portion of the P conductive type base region, a trench is formed to penetrate the N+ conductive type source region and the P conductive type base region and to reach the N− conductive type drift layer, and a gate electrode and a gate oxide film are embedded in the trench. The vertical type MOSFET having the above structure has the number of channels per unit area, which is larger than a planar type power MOSFET, so that a on-state resistance is reduced.
In the SiC semiconductor device having a transistor with a trench gate structure or the like, a high channel mobility and reliability of the gate oxide film are important factor for deciding a device performance. The high channel mobility and the reliability of the gate oxide film are affected by flatness (i.e., smoothness) of a trench side surface. Thus, it is preferably to form the gate oxide film on an inner wall of the trench having flatness as much as possible.
However, the trench side surface of the transistor having the conventional trench gate structure has a surface roughness Ra in a range between a few nanometers and a few tens nanometers. Thus, the flatness is not good.
The present inventors have studied about the above trench gate structure. As a result, the following knowledge is obtained. FIGS. 8A to 8C are diagrams showing states of flatness of a trench side surface when the trench is formed. FIG. 8A is a photograph showing a state of a trench side surface, FIG. 8B is a schematic diagram of FIG. 8A, and FIG. 8C is a cross sectional view seeing along line VIIIC-VIIIC in FIG. 8A. FIGS. 8A and 8B correspond to diagrams seeing the side surface of the trench along an oblique direction. FIG. 8C corresponds to a diagram seeing the trench from above under a condition that an up-down direction of the drawing is defined as a longitudinal direction of the trench. As shown in FIGS. 8A to 8C, a concavity and a convexity are formed on the trench side surface, so that the flatness is worsened. In order to obtain a preferable channel mobility and reliability of the gate oxide film, it is required that the surface roughness Ra of the trench side surface is equal to or smaller than 10 nanometers, and preferably, the surface roughness Ra as the flatness is equal to or smaller than 1 nanometer. Under the present situations, the sufficient channel mobility and reliability of the gate oxide film are not obtained.